Optical ring resonator structures have been developed for optical modulation functions. Most of the proposed (and all of the demonstrated) optical ring resonator modulators are based on silicon-on-insulator (SOI) waveguides using silicon oxide cladding layers. The modulation relies on refractive index changes produced by the absorption of light in the intrinsic region of a p-i-n structure. Since the energy of the light at 1550 nm is less than the energy gap of the Si, then bandgap absorption is not allowed and the absorption must be enabled by the injection of both holes and electrons into the intrinsic region to create free carrier absorption.
One obvious drawback of these configurations is power consumption since relatively high voltages and current are required (the absorption is proportional to the current). Other drawbacks are the low values of absorption for a given current flow and the problem of removing the injected charge when the signal is reduced to zero.